376 



CLASSIFICATION OF VERTEBRATES. 



structures occurring in reptiles ; while there are several features 

 which point to the possibility of their origin from the dermal 

 sense organs of the amphibia. 



SUB-CLASS I. PROTOTHERIA. 



Mammals with a single opening for urogenital system and 

 alimentary canal ; sutures of skull obliterated in the adult ; a 

 well-developed coracoid and episternum ; oviparous. 



ORDER I. MONOTREMATA (ORNITHODELPHIA). 



Prototheria with small corpus callosum and large anterior 

 commissure ; no teeth in the adult ; epipubic bones present ; 

 ribs with capitular head only ; mammary gland without distinct 

 nipple. 



The few existing species of monotremes are restricted to 

 the Australasian region, and the only fossils certainly belonging 

 to the order occur in the pleistocene of 

 Australia. These mammals are remark- 

 able for the large number of sauropsidan 

 features which they present. Besides 

 the characters given in the diagnosis the follow- 

 ing may be added. The ossicula auditus are 

 of a low grade, the malleus being large and the 

 stapes columelliform. In the embryo of the 

 duckbill multituberculate teeth occur, but these 

 are lost before maturity, and the adults of all 

 species are toothless. Lips are lacking, and the 

 jaws form horny beaks. The brain is smooth 

 in Ornithorhynchus, convoluted in Echidna, The 

 testes are abdominal in position ; the left ovary 

 is reduced as in birds, the right lobular. There 

 is a horny perforated spur developed on the hind legs in con- 

 nection with a gland. This spur disappears in the adult female 

 duckbill. 



Family ORNITHORHYNCHIDJE. With duck-like bill, two horny teeth in 

 each jaw; feet pentadactyl, webbed; tail flat; soft, close fur. Ornitho- 

 rhynchus paradoxus, the duckbill of Australia and Tasmania, is the only 



FIG. 355. Em- 

 bryonic teeth of 

 Ornithorhynchus, 

 after Stewart. 



